This invention relates to a particle analyzing device and a method of moving and counting particles in suspension through such a device. More particularly the invention relates to a source of vacuum for drawing a fluid sample, a quantity of a suspension of particles, through a particle counting device having an aperture whose effective impedance is changed with the passage of each particle through said aperture, which vacuum remains substantially constant during its operative cycle. The invention also concerns the repeated movement of the same volume of different fluid blood samples regardless of the altitude at which the device is operated and wherein the magnitude of the vacuum source is self-regulating in that the magnitude of the vacuum generated is only a function of altitude and more particularly the force of gravity. Such a device furthermore repeatedly moves the same volume of different fluid samples in the same interval of time, or operative cycle, at any given altitude and stroke length and wherein said time interval is only a function of altitude. Such a device can also function as an aperture blockage indicator by comparing the actual time interval between the initiation and end of an operative cycle and a known correct time interval at any given altitude thereby obviating the use of a separate optics viewer used to observe the aperture. Although the sample flow rate varies as a function of altitude, since the counting time interval utilized for devices constructed in accordance with this invention is fixed, it is a simple matter of calibration to compensate for such factor. This device which does not utilize a manometer system having a mercury column used as a constant vacuum source or pressure indicator obviates the problems associated with the use of such a toxic substance and is independent of atmospheric variations which affect such a mercury column. Furthermore, since it is manually actuated by an operator, no pumps, electrical power, or other motive power is required to operate its vacuum source and due to its simple design, it is inexpensive to manufacture. Additionally, since the source of the vacuum's driving force is the force of gravity, the driving force does not suffer from any of the problems that are associated with other forces, including temperature, stretching, aging, repeatability, breaking, cracking, and the like.
Furthermore, since the vacuum source operates without any sliding contact during its operative cycle, the problems associated with mechanical friction are totally eliminated thereby resulting in the achievement of high repeatability over long periods of time.
Also since the vacuum remains substantially constant, the necessity of providing an indicator to monitor its pressure is obviated. Furthermore, since the device does not require a continuously generated vacuum, attendant vacuum bottles and supporting hardware are unnecessary.
Manually operative particle analyzing device having a vacuum source which remains substantially constant during its operative cycle are known as shown in U.S. Pat. No. 2,869,078, Fluid Metering Apparatus, issued Jan. 13, 1959 to Coulter et al. Coulter describes the importance of providing "a precise and accurate metering of a predetermined and constant volume of fluid to be tested as it passes the scanning point of a detecting system" (column 1, line 70 to column 2, line 3). This is achieved by utilizing a "manometer system having a mercury column . . . to achieve a substantially unchanging pressure differential in the system during the metering cycle" (column 2, line 20 to line 24). This device suffers from the obvious drawbacks inherent in utilizing such a toxic substance, mercury, as a component of the vacuum source.
A particle counting apparatus which utilizes a suction pump and flow regulator to provide a vacuum source whereby a uniform flow rate is achieved is taught by U.S. Pat. No. 3,654,439, Particle Counting Apparatus Having Automatic Display And Threshold Setting, issued Apr. 4, 1972 to Estelle et al. The pump believed to be utilized was a conventional "fish tank" Bellowfram.RTM. type vacuum pump with a flow regulator, the latter to maintain the vacuum constant. Such a combined vacuum pump and regulator in addition to requiring electric power does not provide as constant a vacuum nor one which is continuously repeatable over time. Furthermore, devices utilizing such types of vacuum sources generally require that the regulator be finely adjusted by such means as a mercury manometer indicator which monitors the pressure of the vacuum source.
A particle counting apparatus which utilizes a vacuum source only during the portions of the operating cycle where vacuum is required and which requires no indicator is taught by U.S. Pat. No. 4,303,337, Apparatus For Detecting Hemoglobin And White Blood Cell Count In Blood, issued Dec. 1, 1981 to James et al. It utilizes an intermittently operable vacuum system, operatable only during the count cycle. This system utilizes a solenoid operated vacuum dispenser coupled to detecting upper and lower limit switches which function to establish operable vacuum limits during the count cycle and to operate an alarm to alert the operator to vacuum malfunctions; it does not provide as constant a vacuum. Since it is electrically powered, it does not have the same advantages as the manually operable device of the present invention as well as requiring electrical components to generate the vacuum which increases its cost. Not being driven by the force of gravity, it lacks the concomitent advantages thereof previously described.